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Replacing Wire with Inexpensive
Plastic Fiber Solutions
IntroductionCommunication with fiber-optics has many advantages overelectrical or “wire”-based interfaces. Unfortunately, fiber hasoften been considered an expensive or exotic solution, lim-ited to high-end applications that absolutely require it. Withthe advent of inexpensive optical devices and Plastic OpticalFiber (POF), this technology can now be implemented atcosts that are competitive with copper wire.

This application note explains the advantages of this low-costfiber-optic solution and ways to implement it using Cypress’CY7B923/933 HOTLink® transceivers. It covers some likelyapplications and a brief explanation of how to use HOTLinkwith optical interfaces. The trade-offs between signaling rateand distance are also addressed.

Included in this application note are detailed design schemat-ics, circuit board artwork, and a complete parts lists that allowimplementation of these low-cost fiber optic links. Alsoincluded are detailed procedures on the handling of plasticoptical fiber used to build these links.

ScopeThis document focuses on the range of performance and dis-tance options possible with low-cost POF. This selectionallows replacement of copper media at distances up to 50mat serial signaling rates of 150-to-160 MBaud. Additional ref-erences are provided for alternative fiber-optic componentsand faster versions of HOTLink that support a much largerrange of distance and data rates.

Advantages of Optical MediaFiber-optic media has numerous advantages over coppercables:

■ Complete electrical isolation

■ Small media size

■ Light weight

■ No radiated emissions from the optical media

■ Immune from external electrical influences

■ High data carrying capacity

These advantages make optical fiber the media of choice forelectrically hostile or sensitive environments. All major issuesof electromagnetic compatibility (radiated emissions, arcing,static discharge, ground loops, ground offsets, stray currents,etc.) disappear.

Fiber-optic links also simplify many design aspects of serialinterfaces:

■ No need for coupling magnetics or capacitors to achieve DC isolation

■ No need for bulky cable shields

■ No transmission-line impedance or crosstalk concerns

Common UsesFiber-optic communications are desirable in many applica-tions:

■ Medical Equipment—A fiber-optic connection provides absolute electrical isolation. This is often necessary for the safety of the patient, or to allow sensitive measurements to occur without the introduction of electrical noise that would upset delicate measuring devices.

■ Heavy machinery—Monitoring and controlling heavy equipment requires very accurate information transfer within the control system and equipment. High levels of electrical interference or ground offsets generated by the equipment can potentially disrupt this communication when used with electrical interfaces.

■ Video—Video requires high bandwidth and moderate dis-tances between the camera and monitoring equipment. The easy handling and termination of plastic optical fiber simplifies cabling and avoids safety and ground offset is-sues. New video standards (such as Digital Video Broad-cast and SMPTE 292M) actually specify fiber-optic inter-faces.

■ In-flight entertainment—Video and audio distribution in air-craft requires few transmitters but many receivers. Due to the sensitive navigation and communications equipment on the aircraft, radiated emissions must be kept to an ab-solute minimum.

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March 11, 1999 Document No. 001-42045 Rev. ** 1

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■ Avionics—Today’s commercial aircraft have in excess of 200,000m of electrical cables, with many used to control the flight processes. These cables add a large amount of weight to the aircraft. Replacing these cables with optical fiber can remove a significant amount of weight from the plane, allowing either greater payload capacity, lower fuel consumption, or greater distance. Commercial aircraft also generate high levels of RF energy for RADAR that must not interfere with other control systems.

■ Equipment back bone—In the most general of applica-tions, connecting board-to-board or chassis-to-chassis, optical fiber offers an electrically secure environment with high bandwidth for data transfer. It can safely be used in hostile process control environments such as nuclear power plants, or other locations where there may be dras-tic differences in potential between the various pieces of equipment.

The physical attributes of optical fiber are also intrinsicallysafer than electrical cables. When cut or damaged there areno shock or fire hazards, and they cannot cause a short cir-cuit. When used with LED-based optical transmitters (or low-power LASERs) there is no optical eye hazard. Plastic (andglass) fiber provides a high degree of flexibility and can carrymore data per equivalent cross-sectional area than coppercables. The use of optical fiber in physically hostile environ-ments, such as water, harsh weather, or explosive gases,presents no issues of safety or loss of functionality.

Implementing a LinkThere are three primary components in a link:

■ Serializer/Deserializer - The quality of the PLLs and driv-ers/ receivers in the serializer/deserializer have a signifi-cant impact on signaling rate and transmission distance. The serializer and electrical driver determine how much jitter is added before the optical transmitter. The deserial-izer extracts both clock and data from the received signal and determines how much jitter the interface can tolerate while continuing to extract correct data. While the Cypress CY7B923/33 HOTLink is the subject of this application note for these functions, additional solutions are also pos-sible using the Cypress CY7C955 SONET/SDH Trans-ceiver and ATM cell processor, or the CY7B951/952 SO-NET Synchronous Transceiver (SST) clock/data separator parts.

■ Media - The media determines many of the issues of sig-naling rate and distance. Greater distances and/or speeds often require improved media and/or optics to overcome losses and dispersion inherent in the fiber.

■ Media driver/receiver - Optical media is driven by either LED or LASER drivers. These convert the serial data be-tween the electrical and optical domains. Characteristics of optical power, spectral purity, and wavelength all effect the dispersion and delivered power. The sensitivity, band-width, and random noise of the optical receiver determines the usable distance and error rate of the link.

Sending data across a link requires more than just stuffingbits in one end and having them show up at the other. Thebits arriving at the deserializer must be sampled with a clockto return them to a digital domain. With slower serial inter-faces, such as audio-spectrum telecommunications modems,this clocking may often be done through an asynchronousoversampling of the received bit stream. At the faster datarates used here, this oversampling becomes quite impracti-cal.

At faster transfer rates the sample clock must operate at thesame rate as the bits in the data stream. While the sampleclock could be delivered on a separate link, this is generallypoor practice. The time skew between data and clock is diffi-cult to manage, and the cost of a second link make this pro-hibitive. Since no sample clock is delivered to the deserializeralong with the data, a clock must be extracted from the datastream. This is accomplished through the use of a high per-formance PLL (Phase Locked Loop) that detects the transi-tions in the serial stream.

Unfortunately, an NRZ (Non-Return to Zero) data stream maycontain few if any transitions, especially when sending data ofmostly one or zero bits. To send data of this type, the dataitself must be modified to force additional transitions into thedata stream. This modification is known as coding.

Data CodingNumerous methods exist to force additional transitions into adata stream. These can generally be broken down into twocategories: scrambling and encoding.

ScramblingScrambling modifies a data stream by merging it with one ormore randomizer polynomials. Scrambling, used in telecom-munications interfaces such as SONET and ATM, is 100%efficient. For every bit in the source data stream, a single bitis sent across the interface.

While this would at first appear as the perfect solution, scram-bling does have its drawbacks. The characteristics of ascrambler are such that the scrambler can be zeroed-out byspecific data patterns. Zero bits transmitted following this aretransmitted without transitions.

A scrambled interface is also somewhat limited in how linkcontrol information is moved across a link. Because the linkefficiency is perfect, all combinations of bits are used to rep-resent data. This requires all link control information to besent as combinations of data characters.





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EncodingThe other method of forcing transitions involves encoding. Inan encoded interface, the source data is modified by mappingthe source data into alternate bit combinations called sym-bols. These symbols are constructed with extra bits that guar-antee a minimum transition density.

Many different forms of encoding are used with serial inter-faces, all having less than 100% efficiency. This means thatmore bits must be transmitted on the serial interface than arepresent in the source data. Encoded interfaces generallyhave link efficiencies ranging from 50-to-95%.

One of the most popular encodings is known as 8B/10B. Thisencoding is used by popular high-speed serial interfacessuch as Fibre Channel, ATM, ESCON™, Gigabit Ethernet,and DVB-ASI. The 8B/10B code maps an 8-bit data characterinto a 10-bit symbol known as a transmission character. Thiscode is optimized for transmission across optical media.

8B/10B encoding is used for a number of reasons:

■ Transition Density - The receiver PLL requires transitions in the received bits to maintain a lock on the data stream 8B/10B encoding forces a large number of transitions into the data stream and prevents continuous strings of ones or zeros from being sent.

■ DC Balance - 8B/10B encoding limits the low frequency content in the data stream. This allows the use of low-cost AC-coupled optical modules.

■ Error Detection - 8B/10B encoded characters follow spe-cific rules that allow many signaling errors to be immedi-ately detected.

■ Special Characters - Special characters are characters that can be included in a data stream that are not decoded as normal data characters. These special characters may be used as general delimiters for controlling an interface (such as video sync, start-of-frame, end-of-frame, etc.) or other generalized commands to the remote site.

When data is encoded, a characteristic known as Run-Length-Limit (RLL) is established. This limit specifies themaximum number of consecutive ones or zeros that canoccur in the transmitted serial bit-stream. For the 8B/10Bcode the run length limit is five.

Serializer/Deserializer - HOTLinkHOTLink is the name for a pair of Cypress devices designedto transmit and receive data across serial interfaces. TheHOTLink transmitter accepts data as 8-bit parallel characters,encodes this data using the 8B/10B encoding rules, and per-forms the parallel-to-serial conversion. This allows the data tobe transmitted across optical (or copper) media to a compan-ion receiver, which extracts clock and data from the serialdatastream, deserializes and decodes the data, and outputs thisas 8-bit parallel characters.

HOTLink OperationHOTLink operates as a pipeline-register extender. It acceptsdata on the rising edge of a synchronous clock operating atthe character rate. For those designs that are not synchro-nous, HOTLink also supports seamless interfacing to eitherclocked or asynchronous FIFOs. This is covered in detail inthe Cypress application notes “Interfacing the CY7B923 andCY7B933 (HOTLink) to Clocked FIFOs” and “Interfacing theCY7B923 and CY7B933 (HOTLink) to a Wide Data ClockedFIFO.”

When operated with FIFOs, data transfer rates much slowerthan the character rate are possible. When the source FIFOcontains little or no data, the HOTLink transmitter automati-cally generates a special “fill” character (known as a K28.5)that may be simply discarded at the receiver. These charac-ters are also used to frame the receiver to the serial bitstream and to maintain synchronization of the link.

Special CharactersThe 8B/10B code used with HOTLink has twelve specialcharacters. These special characters may be transmitted atany time and allow in-band signaling. Except for the previ-ously described fill character (K28.5), the meaning or functionof these special characters is user definable. Table 1 lists the8B/10B names of these special characters, as well as thename and format of these characters when input anddecoded by HOTLink. Additional information on the 8B/10Bcode may be found in the CY7B923/933 HOTLink datasheetand in the Cypress HOTLink User’s Guide.

Table 1. Valid Special Character Codes (SC/D = HIGH)

These special characters are injected into the data stream bypresenting a HIGH level at the SC/D input of the HOTLinktransmitter. When a special character is received and presentin the HOTLink receiver output register, the receiver SC/Doutput is driven HIGH.

For video applications, these special characters can be usedfor horizontal and vertical sync indicators. For other bus-to-bus applications they may be used as control strobes or databorder markers. The user may switch modes of data usingthese markers, e.g., inserting address pointers between datapackets in a point-to-point or networking application.





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Signaling and Data RatesBoth HOTLink and POF components operate over wide sig-naling- rate ranges. HOTLink components are available thatoperate at serial signaling rates of 150-to-400 MBaud. Lowcost POF media, drivers, and receivers are available thatoperate at serial rates of 10-to-160 MBaud. The 155 MBaudrate, supported by both of these component families, is anindustry standard serial rate for ATM and SONET OC-3 tele-communications interfaces.

Unlike a scrambled ATM or SONET interface, an 8B/10Bencoded stream has an overhead of 25%; i.e., to deliver eightdata bits across the serial interface, a total of ten bits mustactually be transmitted and received across the serial inter-face. With a signaling rate of 155 MBaud, the data rate (theamount of user data moved across the interface) is only 124Mbits/second. Since HOTLink uses 8-bit data characters, thesustained byte rate is 15.5 MBytes/second.

Users needing higher data rates can either use unencoded10-bit data (and get 100% of the link bandwidth), or higherspeed versions of HOTLink that support serial signaling ratesup to 400 MBaud. Note that any scheme using unencodeddata is responsible for maintaining sufficient transition densityfor the HOTLink receiver PLL to maintain a lock to the serialdata stream.

Additional information on using scrambler codes is availablein the Cypress application note “Use HOTLink for 9- and 10-bit Data”. Detailed operation and usage of HOTLink is avail-able in the CY7B923/933 datasheet and the Cypress HOT-Link Users Guide.

Initializing the LinkUnlike a parallel interface, high-speed serial interfacesrequire a number of initialization steps before they can reli-ably transfer data. These steps allow the remote receiver tosynchronize itself with the serial transmitter, and to locate thecharacter boundaries in the serial bit-stream.

To achieve synchronization, the local HOTLink serializer mustbe transmitting a continuous stream of characters. The transi-tions in the transmitted bits allow the remote HOTLinkreceiver PLL to achieve both phase and frequency lock to thedata stream. This may require a few thousand bits, but is onlynecessary if the serial stream between the transmitter andreceiver is disrupted.

FramingThe next step is for the HOTLink receiver to locate the begin-ning and ending points of the characters embedded within thedata stream. This process is known as framing, and operatesby detecting a specific sequence of bits within the serial datastream. This specific bit sequence is contained within theK28.5 special character.

Upon power-up, or if framing is ever lost, the HOTLink trans-mitter should be configured to send K28.5 characters untilproper framing is accomplished. If phase and frequency lockhave already been achieved by the HOTLink receiver, fram-ing should occur upon reception of one or two K28.5 charac-ters (depending on the present framing mode of the HOTLinkreceiver). Once proper framing is established, the transmittercan resume sending data.

BISTHOTLink contains a Built-In Self Test (BIST) capability thatcan be used to verify the stability of a link, or simply as amethod of easily testing a link. BIST may be initiated at any-time by asserting the BISTEN input to both the HOTLinktransmitter and receiver. Once enabled, the HOTLink trans-mitter continuously sends a pseudo-random sequence ofcharacters to the receiver, which checks the received charac-ters to verify their accuracy. BIST provides a simple methodof link testing with minimal management control logic.

Evaluation BoardsCY9266 Evaluation boards for HOTLink are available for bothoptical-fiber and copper cable interfaces. These boards maybe interfaced to host systems through either a simple edgeconnector or an IBM OLC-266 compatible connector. TheCY9266-F (and -FX) card supports standard 1X9 opticalmodules from Hewlett Packard and other manufacturers. Theoperating character rate for these cards is set by either anexternal character-rate clock, or through a socketed oscilla-tor. Custom frequencies may be provided by the CypressICD6233 Quixtal programmable oscillator.

The CY9266 contains direct support for BIST, including a 2-digit BIST error-counter display. A single evaluation boardcan be configured to loop the serial stream back to itselfeither locally or through the selected media. Two evaluationboards may be interconnected to form a full duplex link.

Fiber-Optic MediaMedia selection is greatly affected by the required data rateand distance. As the data rate is increased, the bit period isreduced, and as the optical media is lengthened less opticalpower is delivered to the receiver. Both of these cause thestable time within each delivered bit (the “eye” opening) todecrease. When operated with multimode optical media, bothchromatic and modal dispersion also cause the received sig-nal eye to shrink. Similar effects are present with coppercables, caused by the frequency selective attenuation of themedia.





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Four types of fiber are in common use with optical links:

■ Plastic Optical Fiber (POF)

■ Hard Clad Silica (HCS®)

■ Multimode graded index

■ Single-mode

POF is a step-index multimode plastic optical media thattransfers light by optical reflection. This limits its use to rela-tively short operational lengths, and only moderate datarates. It is the lowest cost media to use for optical intercon-nect, and the simplest to connectorize.

Hard Clad Silica (HCS) is a step-index multimode glass fiberthat uses a plastic cladding. The step index of this fiber alsolimits its use to only moderate lengths and data rates. Theglass core has much less attenuation and allows it to be usedwith longer wavelength (1300 nm) optical sources. Its lowerloss also allows it to be used with optical drivers that deliverless power.

Multimode graded-index glass fiber propagates light byrefraction (instead of reflection like step-index fibers). It isdesigned to correct for some modal dispersion that wouldotherwise occur with a step-index fiber. This allows muchlonger distances than multimode step-index fiber. It does nothave the capability to correct for the chromatic dispersion thatoccurs with LED drivers. This media is also used with manyshortwave LASER drivers.

Single-mode glass is the highest performance optical fiber. Itis used for the highest data rates and longest distances. Likemultimode graded-index fiber, it also operates by refraction.Because of its narrow 9-mm core diameter, this fiber is onlyusable with 1300-nm optical sources. This core diameter alsorequires very high-tolerance connectors to insure properalignment.

A fifth type of optical fiber has recently been developed, but isnot yet in common use. This fiber is known as GIPOF orGraded-Index Plastic Optical Fiber. This fiber corrects formodal dispersion and offers the promise of longer link lengthswith a plastic media, while preserving the simple assemblycharacteristics of step-index POF.

Optical ReceiversAn optical receiver converts optical pulses into electricalpulses. In these receivers, light is detected by a PIN photo-diode. The changes in light cause small changes in currentthrough the photodiode. These current changes are then con-verted to voltage changes in a transimpedance preamplifier,followed by multiple limiter gain stages to convert the weaksignal to full digital levels.

The limiter stages of optical receivers are either AC-coupledor direct coupled. The selection of the coupling type is basedprimarily on the type of data that the receiver must deal with.With scrambled interfaces that may contain long runs of onesor zeros, the optical receiver must be direct coupled. Thisrequires the fiber-optic receiver to detect DC levels or edgesto determine the proper logic state during these long periodsof inactivity.

Unfortunately, direct-coupled optical receivers suffer fromnumerous drawbacks. Direct coupling decreases the sensitiv-ity of a digital fiber-optic receiver, since it allows low-fre-quency flicker noise from amplifiers to be presented to thereceiver's comparator input. Any undesired signals coupledto the comparator reduce the signal-to-noise ratio at this criti-cal point in the circuit, and reduce the sensitivity of the fiber-optic receiver.

When operated with encoded data, the receiver’s PIN pre-amplifier should be AC-coupled to a limiting amplifier andcomparator as shown in Figure 1. AC-coupled fiber-opticreceivers tend to be lower in cost, much simpler to design,provide better sensitivity, and contain fewer components than

their direct-coupled counterparts. Another problem associ-ated with direct-coupled receivers is the accumulation of DC-offset. With direct coupling, the receiver's limiter stagesamplify the effects of undesirable offsets and voltage driftsdue to temperature changes or aging. These amplified offsetsare eventually applied to the comparator and result inreduced sensitivity of the fiber-optic receiver and can appearas Duty Cycle Distortion (DCD). Problems with DC-offset canbe avoided by constructing the receiver as shown in Figure 1.

Advantages of Encoded Run-Limited DataAs the data rate of a fiber-optic link increases, the reasons forencoding the data become very compelling. The encodedcharacters which replace the original data characters areselected so that the encoded data is compatible with simple,highly sensitive, AC-coupled fiber-optic receivers. Encodingenables the construction of optimal fiber-optic receiverswhich are limited by the random noise inherent in thereceiver’s first amplifier stage. Optimal noise-limited AC-cou-pled receivers can provide very low error-rates in applicationswith long optical fibers.





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Figure 1. Simplified +5 Volt ECL (PECL) Compatible Fiber-Optic Receiver Block Diagram for High Data Rate Applications with Encoded Data

Optical Link DesignThe CY7B923/933 HOTLink transmitter and receiver aredesigned to interface to optical components at PECL (Posi-tive ECL) signal levels. These signal levels are used by allmanufacturers of high-speed optical modules. Guidelines forinterconnecting HOTLink to optical modules are located inthe “CY9266 HOTLink Evaluation Board User’s Guide” andthe Cypress HOTLink User’s Guide.

Only One Transceiver Design NeededThe schematics and artwork in this application note docu-ment a fiber-optic transceiver design that can accept variousHewlett Packard LED transmitters and PIN-diode pre-amplifi-ers. This transceiver both provides and accepts differentialPECL signal levels and is directly compatible with HOTLink.This transceiver design may be embedded into a wide rangeof products to provide very low-cost data communicationsolutions. To simplify the integration of this transceiver intoHOTLink- based communications links, the complete fabrica-tion artwork for the fiber-optic transceiver is available as aself extracting archive (Gerber photoplot and drill files) fromhttp://www.hp.com/HP-COMP/fiber/furball.exe.

Without changing the form-factor or printed circuit design, thefiber-optic transceiver documented in this application notemay be populated with alternate component sets that cancommunicate over:

■ Large-core step-index POF

■ Step-index HCS fiber

■ Multimode graded-index glass fiber

■ Single-mode glass fiber

This makes this single design suitable for an extremely widerange of data communication applications.

Distances Limits at 160 MBaud Signaling RateThe simple transceiver design recommended in this applica-tion note can be used to address a very wide range of dis-tances and system cost targets. While the focus of thisapplication note is on the 50-meter POF transceiver high-lighted in Table 2, alternate combinations of optical fiber andfiber-optic transceiver components in this same table can beused to address specific link requirements. Transceiversbased on these components require no adjustments whenoperated within the distance ranges listed in Table 2.





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Table 2. Low Cost Optical Components Compliant With Figure 4

SafetyThe HFBR-15X7 LED-based optical transmitter meets IEC-825-1 Class 1 safety standards for optical sources. Thatmeans that it poses little or no hazard to human vision andthat no external or special measures are required for eye pro-tection.

Simple PECL Compatible LED TransmitterA high-performance low-cost PECL-compatible fiber-optictransmitter design is shown in Figure 2. This design looksdeceptively simple but has been highly developed to deliverthe best performance achievable with a wide range ofHewlett Packard LED transmitters. The recommended driverfor the LED is also very inexpensive, since the 74ACTQ00gate used to current modulate the various optical transmitterscan typically be obtained for under $0.40. When used withthe recommended component values listed in Table 3, thetransmitters and receivers listed in Figure 2 can be used toaddress a wide range of applications.

Simple High Sensitivity PECL Compatible ReceiverA very simple PECL-compatible receiver, with excellent sen-sitivity and suited for a wide range of applications, is shown inFigure 3. A single low cost 10H116 ECL line receiver, obtain-able for less than $1.00, is used to amplify and digitize theoutput of the Hewlett Packard PIN-diode pre-amp componentwhich functions as the receiver’s first stage. The third sectionof the 10H116 integrated circuit is configured to provide hys-teresis such that when no light is applied to the optical detec-tor, the digital output of the receiver does not chatter.

Figure 2. +5V ECL (PECL) Compatible 160 MBd Fiber-Optic Transmitter





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Figure 3. +5V ECL (PECL) Compatible 160 MBd Fiber-Optic Receiver

Table 3. Optical Driver Component Values

A Complete Fiber-Optic Transceiver SolutionFigure 4 shows the schematic for a complete fiber-optictransceiver. This transceiver may be constructed on a 1" x1.97" printed circuit board, using surface mount components.When this transceiver is operated at 160 MBaud through 50mof 1 mm diameter plastic optical fiber (numerical aperture of0.35), it provides a typical eye opening of 3.6 ns with verylowerror rates.

Designers interested in inexpensive optical links are encour-aged to embed the complete fiber-optic transceiver describedin Figure 4 into their products. This design matches the elec-trical functions of industry standard 1X9 transceiver modules,except for the lack of a signal detect output. Since the HOT-Link receiver RVS output provides a direct indication of linktransmission errors, and can therefore be used to qualify alink, this status indicator is not generally necessary.

For those designs that do require a signal detect output, the10H116 can be replaced by a ML6622 at only slightly higherlink cost. The schematic and artwork for this modifiedreceiver can be found in the Hewlett Packard application note1123.

The design in Figure 4, when implemented on the board lay-out documented in Appendix B, can be directly inserted intoboards designed for industry standard fiber-optic transceivermodules with a 1X9 footprint (including the HOTLinkCY9266-F evaluation board) and used as a lower-cost alter-native in industrial, medical, telecom, and proprietary datacommunication applications.

Error Rates and Noise ImmunityThe probability that a fiber-optic link will make an error isrelated to the optical receiver’s own internal random noise, itsability to reject noise originating from the system in which it isinstalled, and the jitter tolerance of the clock and data recov-ery (CDR) circuit. The total noise present in any fiber-opticreceiver is normally the sum of the PIN diode, preamplifier,and the host system’s electrical noise.





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Figure 4. Lowest Cost 160 MBaud Fiber-Optic Transceiver

Figure 5. Receiver Signal-to-Noise Ratio vs. Probability of Error (BER)

As the optical signal applied to the receiver increases, theprobability that the receiver’s total noise will alter the datadecreases. Small increases in the receiver’s signal-to-noiseratio result in a very sharp reduction in the probability of error.Figure 5 shows that the receiver’s probability of error isreduced by six orders of magnitude from 1x10–9 to 1x10–15when the receiver’s signal-to-noise ratio improves from 12:1to 15.8:1.

At any fixed temperature, the total of the receiver’s randomnoise (flicker, thermal, shot) and the host system’s noise canbe assumed to be a constant. While the most obvious way toreduce the probability of an error is to increase the amplitudeof the optical signal applied to the receiver, a less obvioustechnique is to improve the receiver’s ability to reject electri-cal noise from the system. The fiber-optic receivers docu-mented in this application note have sufficient noise immunityto be used in most systems without electrostatic shielding.The Hewlett Packard PIN-diode preamplifiers (used in thereceiver’s first stage) are physically small hybrid circuits, anddo not function as particularly effective antennas. Forextremely noisy applications, Hewlett Packard offers PINdiode preamplifiers in electrically conductive plastic or allmetal packages. The overwhelming majority of the fiber-opticapplications do not require conductive plastic or metalreceiver housings.





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Signal Terminations and Power Supply Filtering RequirementsThe most overlooked source of noise is usually the host system’s power supply. This supply normally powers the fiber-opticreceiver, the fiber-optic transmitter, the serializer, CDR, deserializer, and a system comprised of relatively noisy digital circuits.The simple and inexpensive power supply filter in Figure 6 has been proven to work in a wide range of system applications. A fil-ter of this type is normally sufficient to protect the fiber-optic receiver from very noisy host systems.

Figure 6. Recommended Power Supply Filter and +5V ECL (PECL) Signal Terminations for the Cypress HOTLink

When used with HOTLink, the power supply filter and termi-nations shown in Figure 6 are recommended. In this environ-ment, the transceiver in Figure 4 has been proven to provideexcellent performance. The capacitor values are chosenbased on series resonance considerations as discussed inthe Cypress application note “Using Decoupling Capacitors.”

Printed Circuit ArtworkThe performance of any fiber-optic transceiver is partiallydependent on the layout of the printed circuit board on whichthe transceiver is constructed. To achieve the link perfor-mance listed in Table 2 for the transceiver shown in Figure 4,designers are encouraged to embed the printed circuit art-work provided in Appendix B of this application note.

Parts ListThe PECL-compatible fiber-optic transceiver documented inthis application note is simple, inexpensive, and uses only afew external components. For those interested in implement-ing this transceiver design, a complete parts list is provided inAppendix A. All of the components listed in the parts list wereselected to assure that they are compatible with the artworkshown in Appendix B.





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Plastic Optical CablesPlastic optical fiber cables are both low in cost and simple toassemble. An individual connector can be assembled inaround 45 seconds. The optical fiber is protected by a jacketthat makes it durable and resistant to damage. The normalmechanical stresses encountered during installation, or thelimited mechanical movement of usage, have little effect onits performance.

The assembly of plastic fiber-optic cables requires no com-plex crimping tools or polishing techniques. The assembly ofthe duplex Hewlett Packard HFBR-4531 connector for POF isdescribed in Appendix C. These same connectors may beused for simplex fiber links, with minor changes to the assem-bly process. Additional information on these connectors isavailable through Hewlett Packard.

ConclusionThe fiber-optic transceiver design documented in this appli-cation note can be used to implement data communicationlinks that operate at high data rates and provide better noiseimmunity than is possible with copper cables. When fiber-optic media and transceivers are used in place of conven-tional copper cable alternatives, it is possible to build indus-trial, medical, telecom, or proprietary communication linkswith much higher tolerance for noise transients (caused byutility power switch gear, motor drives, lightning strikes, etc.),while emitting no EMI, RFI, or other electrical disturbance oftheir own.

These same low-cost optical transceivers can be coupledwith HOTLink as a general replacement for both serial andparallel copper interfaces. When used with plastic opticalfiber, the cost for the entire solution is competitive with copperimplementations. By embedding the solution shown in thisapplication note, designers can quickly develop high-speednoise-immune optical communication links with minimaldevelopment costs.

References1. Cypress Semiconductor Application Note, "Interfacing the

CY7B923 and CY7B933 (HOTLink) to Clocked FIFOs"

2. Cypress Semiconductor Application Note, "Interfacing the CY7B923 and CY7B933 (HOTLink) to a Wide Data Clocked FIFO"

3. Cypress Semiconductor Application Note, "Use HOTLink for 9- and 10-Bit Data"

4. Cypress HOTLink User’s Guide

5. Cypress Semiconductor Application Note, "Using Decou-pling Capacitors"

6. Hewlett Packard application bulletin 78, "Low Cost Fiber-Optic Links for Digital Applications up to 155 MBd"

7. Hewlett Packard Technical Data, "Plastic Optical Fiber and HCS Fiber Cable and Connectors for Versatile Link"

8. Hewlett Packard Technical Data, "Crimpless Connectors for Plastic Optical Fiber and Versatile Link"

9. Hewlett Packard Application Note 1123, "Inexpensive 20 to 160 MBd Fiber-Optic Solutions for Industrial, Medical, Telecom, and Proprietary Data Communication Applica-tions"

10. Hewlett Packard Application Note 1066, "Fiber-Optic So-lutions for 125 MBd Data Communication Applications at Copper Wire Prices"





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Appendix A. Parts List





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Appendix B. Printed Circuit Artwork





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Appendix C. Assembly of POF Connectors and Cables

Assembly of POF Connectors and CablesPOF media and connectors are both low in cost and verysimple to assemble. The Hewlett Packard HFBR-4531 con-nector documented here may be used for either simplex orduplex cable assemblies. The following instructions are pri-marily for a duplex (two-fiber) assembly.

Figure 7. HP Duplex Crimpless POF Connector

Only limited supplies or tools are needed to assemble a POFcable:

■ Plastic optical cable

■ Wire cutters or scissors

■ 16 gauge (AWG) wire strippers

■ Hewlett Packard Crimpless POF connectors ( Figure 7)Hewlett Packard polishing tool ( Figure 8)

■ 600-grit abrasive paper

Figure 8. Fiber Polishing Fixture

Step 1: Stripping the FiberAfter separating the two jacketed fibers by 100–150mm (4–6inches), use the wire strippers to remove approximately 7 mm(0.3 inches) of the fiber jacket. For a duplex assembly, bothfibers should be stripped approximately the same amount.The stripped fiber is shown in Figure 9.

Figure 9. Fiber With Jacket Removed

Step 2: Putting on the ConnectorThe HFBR-4531 connector is a hermaphroditic assembly thatallows two simplex connectors to be used to make a duplexconnector. Insert each fiber into the ferrule of a connector-half until the fiber jacket stops it. This is shown in Figure 10.The fiber should protrude at least 1.5 mm (0.06 inches)beyond the end of the ferrule.

Next the two connector halves are assembled to each other.This locks the fibers in place prior to finishing the ends ofeach fiber. Place one connector-half on top of the other sothat the non-ferrule half of each connector is over the ferrulehalf of the opposite connector-half.

Manually press the connector-halves together over the centerof each fiber. Secure the fiber by latching the connectorhalves together.

Figure 10. Fibers Inserted Into Connector Ferrules





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Appendix C. Assembly of POF Connectors and Cables (continued)

Figure 11. Assembled Duplex Connector

Step 3: Trimming and PolishingAny fiber protruding more than 1.5 mm (0.06 inches) from theconnector end should be trimmed off with a pair of scissors orcutters ( Figure 11), then insert the connector into the polish-ing fixture with the trimmed fiber protruding from the bottomof the fixture as shown in Figure 12. Press the polishing tooldown on the 600-grit abrasive paper and polish the fiberusing a figure eight motion as shown in Figure 13.

Figure 12. Assembled Connector And Polishing Fixture

Figure 13. Fiber Polishing

Step 4: FinishingAdditional polishing on Hewlett Packard 3 mm pink lapping-film can achieve a 2 dB improvement over just 600-grit abra-sive paper ( Figure 13).

Figure 14. Finished Connector Assembly





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